Adherence to clinical appointment schedules by patients on antiretroviral therapy (ART) is necessary for the prevention of medication interruptions, viral rebound, and the development of drug resistance. An observational study conducted in 2010, Enablers and Adherence to Antiretroviral Therapy in Cambodia, sought to identify factors that predict on-time clinical appointment attendance by patients on ART. Clinical data, including appointment attendance across five consecutive visits, were collected from hospital records on a random sample of ART patients at government referral hospitals (RHs) in Battambang Province, Cambodia. Interviews were conducted to obtain quantitative information from patients on their experiences of support services provided by local NGOs and RHs. This information was used to identify ART patient care and support system factors that could potentially enable patients to adhere to clinical appointment schedules. These factors included adherence counseling, support groups, home-based care (HBC) services, and support provided for transportation to ART appointments. Bivariate and multivariable logistic regression analysis was done to assess relationships between system variables and the ART appointment adherence outcome. Of the 289 study participants, 20.4% had missed at least one of the five appointments in the study period. The hospital source of ART services, participation in a hospital-based support group, receiving a loan from a microfinance institution, and the frequency of adherence counseling were found to be associated with ART appointment adherence. No significant associations were found between other support system factors such as HBC, transportation support, food/monetary support, and appointment adherence.